Difference between revisions of "Colon (anatomy)" - New World Encyclopedia

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   Caption2    = Front of abdomen, showing surface markings for liver, stomach, and large intestine. |
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The '''colon''' is the longest portion of the [[large intestine]] of [[vertebrate]]s; in [[mammal]]s, this section of the [[gastrointestinal tract]] extends from the [[cecum]] to the [[rectum]]. The colon often is incorrectly used in the meaning of the whole large intestine altogether; it is really only the biggest part of the large intestine. A storage tube for solid wastes, the main function of the colon appears to be extraction of [[water]] and salts from [[feces]].  
 
The '''colon''' is the longest portion of the [[large intestine]] of [[vertebrate]]s; in [[mammal]]s, this section of the [[gastrointestinal tract]] extends from the [[cecum]] to the [[rectum]]. The colon often is incorrectly used in the meaning of the whole large intestine altogether; it is really only the biggest part of the large intestine. A storage tube for solid wastes, the main function of the colon appears to be extraction of [[water]] and salts from [[feces]].  
−
 
+
{{toc}}
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+
While chemical digestion does not take place in the colon—there are no digestive [[enzyme]]s produced after the [[small intestine]]—there is a [[symbiosis|symbiotic]] relationship with [[bacteria]] that helps break down fiber, producing some nourishment for the body. Herbivores (like [[horse]]s and [[rabbit]]s), which depend on microbial fermentation, have a larger colon than omnivores (like [[pig]]s and [[human]]s), which in turn is larger than that of carnivores (such as [[dog]]s and [[cat]]s) (Bowen 2000).
  
 
==Anatomy==
 
==Anatomy==
−
[[Image:Stomach colon rectum diagram.svg|right]]
+
The ''large intestine'' is the terminal, tubular portion of the [[gastrointestinal tract]] (gut) of [[vertebrate]]s prior to the [[anus]] or [[cloaca]]. Three major portions of the large intestine generally are recognized in [[mammal]]s: ''caecum'' (blind-ended pouch), ''colon'' (majority of the length of the intestine), and ''rectum'' (short, terminal segment) (Bowen 2000). The location of the parts of the colon are either in the abdominal cavity or behind it in the [[retroperitoneum]]. The colon in those areas is fixed in location.  
−
The ''large intestine'' is the terminal, tubular portion of the [[gastrointestinal tract]] (gut) of [[vertebrate]]s prior to the [[anus]] or [[cloaca]]. Three major portions of the large intestine generally are recognized in [[mammal]]s: ''caecum'' (blind-ended pouch), ''colon'' (majority of the length of the intestine), and ''rectum'' (short, terminal segment) (Bowen 2000).  
 
  
−
The location of the parts of the colon are either in the abdominal cavity or behind it in the [[retroperitoneum]]. The colon in those areas is fixed in location. 
+
Arterial supply to the colon of humans comes from branches of the superior and inferior mesenteric arteries. Flow between these two systems communicates via a "marginal artery" that runs parallel to the colon for its entire length. Historically, it has been believed that the ''arc of Riolan'', or the ''meandering mesenteric artery'' (of Moskowitz), is a variable vessel connecting the proximal superior mesenteric artery (SMA) to the proximal inferior mesenteric artery (IMA). However, recent studies conducted with improved imaging technology have questioned the actual existence of this vessel, with some experts calling for the abolition of the terms from future medical literature.
−
 
 
−
Arterial supply to the colon of mammals comes from branches of the superior and inferior mesenteric arteries. Flow between these two systems communicates via a "marginal artery" that runs parallel to the colon for its entire length. Historically, it has been believed that the ''arc of Riolan'', or the ''meandering mesenteric artery'' (of Moskowitz), is a variable vessel connecting the proximal superior mesenteric artery (SMA) to the proximal inferior mesenteric artery (IMA). However, recent studies conducted with improved imaging technology have questioned the actual existence of this vessel, with some experts calling for the abolition of the terms from future medical literature.
 
  
 
Venous drainage usually mirrors colonic arterial supply, with the [[inferior mesenteric vein]] draining into the [[splenic vein]], and the [[superior mesenteric vein]] joining the splenic vein to form the [[portal vein]], which then enters the [[liver]].
 
Venous drainage usually mirrors colonic arterial supply, with the [[inferior mesenteric vein]] draining into the [[splenic vein]], and the [[superior mesenteric vein]] joining the splenic vein to form the [[portal vein]], which then enters the [[liver]].
  
−
Lymphatic drainage from the entire colon and proximal two-thirds of the rectum is to the paraortic nodes, which then drain into the [[cisterna chyli]]. The lymph from the remaining rectum and anus can either follow the same route, or drain to the internal illiac and superficial inguinal nodes. The [[dentate line]] only roughly marks this transition.
+
Lymphatic drainage from the entire colon and proximal two-thirds of the rectum is to the paraortic nodes, which then drain into the [[cisterna chyli]]. The lymph from the remaining rectum and anus can either follow the same route, or drain to the internal illiac and superficial inguinal nodes. The [[dentate line]] only roughly marks this transition.
  
−
==Portions of the mammalian colon==
+
==Portions of the human colon==
−
In [[mammal]]s, the colon consists of the ascending colon, transverse colon, the descending colon, and the [[sigmoid colon]]. The colon from [[cecum]] to the splenic flexure (the junction between the transverse and descending colon) is also known as the right colon. The remainder is known as the left colon.
+
[[Image:Dickdarm-Schema.svg|right|thumb|240px|Portions of the human colon (1, ascending colon; 2, traverse colon; 3, descending colon; 4, sigmoid colon)]]
 +
In [[mammal]]s, the colon consists of the ascending colon, transverse colon, and the descending colon, and in some mammals, including humans, a fourth section, the [[sigmoid colon]]. The colon from the [[cecum]] to the splenic flexure (the junction between the transverse and descending colon) is also known as the right colon. The remainder is known as the left colon.
  
 
=== Ascending colon ===
 
=== Ascending colon ===
−
The ascending colon, on the right side of the abdomen, is about 12.5 centimeters long. It is the part of the colon from the cecum to the hepatic flexure (the turn of the colon by the liver). It is [[retroperitoneal]] in most humans; that is, in the anatomical space behind (retro) the abdominal cavity. In ruminant grazing animals, the cecum empties into the spiral colon.
+
The ascending colon, on the right side of the abdomen, is about 12.5 centimeters long in humans. It is the part of the colon from the cecum to the hepatic flexure (the turn of the colon by the liver). It is [[retroperitoneal]] in most humans; that is, in the anatomical space behind (retro) the abdominal cavity. In ruminant grazing [[animal]]s, the cecum empties into the spiral colon.
  
−
Anteriorly, the ascending colon it is related to the coils of small intestine, the right edge of the greater omentum, and the anterior abdominal wall. Posteriorly, it is related to the iliacus, the iliolumbar ligament, the quadratus lumborum, the transverse abdominis, the diaphragm at the tip of the last rib, the lateral cutaneous, ilioinguinal, and iliohypogastric nerves, the iliac branches of the iliolumbar vessels, the fourth lumbar artery, and the right [[kidney]].
+
Anteriorly, the ascending colon is related to the coils of small intestine, the right edge of the greater omentum, and the anterior abdominal wall. Posteriorly, it is related to the iliacus, the iliolumbar ligament, the quadratus lumborum, the transverse abdominis, the diaphragm at the tip of the last rib, the lateral cutaneous, ilioinguinal, and iliohypogastric nerves, the iliac branches of the iliolumbar vessels, the fourth lumbar artery, and the right [[kidney]].
  
 
The ascending colon is supplied by parasympathetic fibers of the vagus nerve (CN X).
 
The ascending colon is supplied by parasympathetic fibers of the vagus nerve (CN X).
  
−
Arterial supply of the ascending colon comes from the [[ileocolic artery]] and [[right colic artery]], both branches of the SMA. While the ileocolic artery is almost always present, the right colic can be absent in five to fifteen percent of individuals.
+
Arterial supply of the ascending colon comes from the [[ileocolic artery]] and [[right colic artery]], both branches of the SMA. While the ileocolic artery is almost always present, the right colic can be absent in five to fifteen percent of individuals.  
−
 
 
 
=== Transverse colon ===
 
=== Transverse colon ===
−
The transverse colon is the part of the colon from the [[hepatic flexure]] (the turn of the colon by the [[liver]]) to the [[splenic flexure]] (the turn of the colon by the [[spleen]]). The transverse colon hangs off the [[stomach]], attached to it by a wide band of [[Tissue (biology)|tissue]] called the [[greater omentum]]. On the [[posterior]] side, the transverse colon is connected to the [[posterior abdominal wall]] by a [[mesentery]] known as the [[transverse mesocolon]].
+
The transverse colon is the part of the colon from the [[hepatic flexure]] (the turn of the colon by the [[liver]]) to the [[splenic flexure]] (the turn of the colon by the [[spleen]]). The transverse colon hangs off the [[stomach]], attached to it by a wide band of [[Tissue (biology)|tissue]] called the [[greater omentum]]. On the [[posterior]] side, the transverse colon is connected to the [[posterior abdominal wall]] by a [[mesentery]] known as the [[transverse mesocolon]].
  
−
The transverse colon is encased in [[peritoneum]], and is therefore mobile (unlike the parts of the colon immediately before and after it). More cancers form as the [[large intestine]] goes along and the contents become more solid (water is removed) in order to form [[feces]].
+
The transverse colon is encased in [[peritoneum]], and is therefore mobile (unlike the parts of the colon immediately before and after it). More [[cancer]]s form as the [[large intestine]] goes along and the contents become more solid (water is removed) in order to form [[feces]].  
  
−
The proximal two-thirds of the transverse colon is perfused by the [[middle colic artery]], a branch of [[superior mesenteric artery]], while the latter third is supplied by branches of the [[inferior mesenteric artery]]. The "watershed" area between these two blood supplies, which represents the embryologic division between the [[midgut]] and [[hindgut]], is an area sensitive to ischemia.
+
The proximal two-thirds of the transverse colon is perfused by the [[middle colic artery]], a branch of [[superior mesenteric artery]] (SMA), while the latter third is supplied by branches of the [[inferior mesenteric artery]] (IMA). The "watershed" area between these two blood supplies, which represents the embryologic division between the [[midgut]] and [[hindgut]], is an area sensitive to ischemia.
  
 
=== Descending colon ===
 
=== Descending colon ===
−
The descending colon is the part of the colon from the splenic flexure to the beginning of the sigmoid colon. It is [[retroperitoneal]] in two-thirds of humans. In the other third, it has a (usually short) mesentery. Arterial supply comes via the [[left colic artery]].
+
The descending colon is the part of the colon from the splenic flexure to the beginning of the sigmoid colon. It is [[retroperitoneal]] in two-thirds of humans. In the other third, it has a (usually short) mesentery. Arterial supply comes via the [[left colic artery]].
  
−
=== Sigmoid colon ===<!-- This section is linked from [[Sigmoid]] —>
+
=== Sigmoid colon ===
−
[[Image:Intestine.png|225px|thumb|right|Diagram of the Human Intestine.]]
+
[[Image:Stomach colon rectum diagram.svg|right]]
−
The '''sigmoid colon''' is the part of the [[large intestine]] after the descending colon and before the [[rectum]]. The name ''sigmoid'' means S-shaped (see [[wiktionary:sigmoid|sigmoid]]). The walls of the sigmoid colon are muscular, and contract to increase the pressure inside the colon, causing the [[feces|stool]] to move into the rectum.
+
The sigmoid colon is the part of the [[large intestine]] after the descending colon and before the [[rectum]]. The name ''sigmoid'' means S-shaped. The walls of the sigmoid colon are muscular and contract to increase the pressure inside the colon, causing the [[feces|stool]] to move into the rectum.
  
−
The sigmoid colon is supplied with blood from several branches (usually between 2 and 6) of the [[sigmoid arteries]], a branch of the IMA. The IMA terminates as the [[superior rectal artery]].
+
The sigmoid colon is supplied with blood from several branches (usually between two and six) of the [[sigmoid arteries]], a branch of the IMA. The IMA terminates as the [[superior rectal artery]].
  
 
[[Sigmoidoscopy]] is a common diagnostic technique used to examine the sigmoid colon.
 
[[Sigmoidoscopy]] is a common diagnostic technique used to examine the sigmoid colon.
  
 
=== Redundant colon ===
 
=== Redundant colon ===
−
One variation on the normal anatomy of the colon occurs when extra loops form, resulting in a longer than normal organ. This condition, referred to as '''redundant colon''', typically has no direct major health consequences, though rarely [[volvulus]] occurs resulting in obstruction and requiring immediate medical attention.<ref>{{cite web
+
One variation on the normal anatomy of the colon occurs when extra loops form, resulting in a longer than normal organ. This condition, referred to as ''redundant colon'', typically has no direct major health consequences, though rarely [[volvulus]] occurs resulting in obstruction and requiring immediate medical attention (Nall 2015).
−
|url=http://www.mayoclinic.com/health/redundant-colon/AN00239/rss=1
 
−
|title=Redundant colon: A health concern?
 
−
|accessdate=2007-06-11 | date=2006-10-13
 
−
|author=Mayo Clinic Staff|work=Ask a Digestive System Specialist|publisher=MayoClinic.com
 
−
}}</ref>  A significant indirect health consequence is that use of a standard adult [[colonoscope]] is difficult and in some cases impossible when a redundant colon is present, though specialized variants on the instrument (including the pediatric variant) are useful in overcoming this problem.<ref>{{cite journal
 
−
| last = Lichtenstein | first = Gary R.
 
−
| coauthors = Peter D. Park, William B. Long, Gregory G. Ginsberg, Michael L. Kochman
 
−
| title = Use of a Push Enteroscope Improves Ability to Perform Total Colonoscopy in Previously Unsuccessful Attempts at Colonoscopy in Adult Patients
 
−
[[
 
−
== Abdul-Sattar Al-Najjar- Translator ==]]'''Bold text'''
 
  
−
| journal = The American Journal of Gastroenterology | volume = 94 | issue = 1 | pages = 187
+
A significant indirect health consequence is that use of a standard adult [[colonoscope]] for endoscopic examination is difficult and in some cases impossible when a redundant colon is present, though specialized variants on the instrument (including the pediatric variant) are useful in overcoming this problem (Lichtenstein et al. 1998).
−
| date = 18 August 1998 | pmid = 9934753 | accessdate = 2007-06-12
 
−
| doi = 10.1111/j.1572-0241.1999.00794.x
 
−
}} ''Note:Single use PDF copy provided free by [[Blackwell Publishing]] for purposes of Wikipedia content enrichment.''</ref>
 
  
 
== Function ==
 
== Function ==
−
The [[large intestine]] comes after the [[small intestine]] in the digestive tract and measures approximately 1.5 [[meters]] in [[length]]. Although there are differences in the large intestine between different [[organisms]], the large intestine is mainly responsible for storing waste, reclaiming water, maintaining the [[water balance]], and absorbing some [[vitamins]], such as [[vitamin K]].
+
Although there are differences in the [[large intestine]] between different [[organisms]], the large intestine is mainly responsible for storing waste, reclaiming water, maintaining the [[water balance]], and absorbing some [[vitamin]]s, such as [[vitamin K]].
 +
 
 +
By the time the [[chyme]] has reached this tube, almost all [[nutrients]] and 90 percent of the water have been absorbed by the body. At this point some [[electrolyte]]s like [[sodium]], [[magnesium]], and [[chloride]] are left as well as indigestible [[carbohydrate]]s known as [[dietary fiber]]. As the chyme moves through the large intestine, most of the remaining [[water]] is removed, while the chyme is mixed with [[mucus]] and [[bacteria]] known as [[gut flora]], and becomes feces. The bacteria break down some of the [[fiber]] for their own nourishment and create [[acetate]], [[propionate]], and [[butyrate]] as waste products, which in turn are used by the cell lining of the colon for nourishment. This is an example of a [[symbiotic relationship]] and provides about one hundred [[calories]] a day to the human body.  
  
−
By the time the [[chyme]] has reached this tube, almost all [[nutrients]] and 90% of the water have been absorbed by the body. At this point some [[electrolyte]]s like [[sodium]], [[magnesium]], and [[chloride]] are left as well as indigestible [[carbohydrates]] known as [[dietary fiber]]. As the chyme moves through the [[large intestine]], most of the remaining [[water]] is removed, while the chyme is mixed with [[mucus]] and [[bacteria]] known as [[gut flora]], and becomes feces. The bacteria break down some of the [[fiber]] for their own nourishment and create [[acetate]], [[propionate]], and [[butyrate]] as waste products, which in turn are used by the cell lining of the colon for nourishment. This is an example of a [[symbiotic relationship]] and provides about one hundred [[calories]] a day to the body. The large intestine produces no digestive [[enzymes]] &mdash; [[chemical digestion]] is completed in the [[small intestine]] before the chyme reaches the large intestine. The [[pH]] in the colon varies between 5.5 and 7 (slightly [[acidic]] to neutral).
+
The large intestine produces no digestive [[enzyme]]s&mdash;[[chemical digestion]] is completed in the [[small intestine]] before the chyme reaches the large intestine. The [[pH]] in the colon varies between 5.5 and 7 (slightly [[acidic]] to neutral).
  
 
==Pathology==
 
==Pathology==
−
 
+
[[Image:illu_intestine.jpg|right|thumb|340px|Intestines]]
−
 
 
 
There are a number of diseases or disorders of the colon:
 
There are a number of diseases or disorders of the colon:
 
* [[Angiodysplasia]] of the colon
 
* [[Angiodysplasia]] of the colon
Line 107: Line 91:
  
 
==References==
 
==References==
−
{{reflist}}
+
* Bowen, R. 2006. [http://www.vivo.colostate.edu/hbooks/pathphys/digestion/largegut/index.html The large intestine: Introduction and index] ''Colorado State''. Retrieved March 28, 2017.
−
 
+
*&mdash;&mdash;&mdash;. 2000. [http://www.vivo.colostate.edu/hbooks/pathphys/digestion/largegut/anatomy.html Gross and microscopic anatomy of the large intestine] ''Colorado State''. Retrieved March 28, 2017.
−
* Bowen, R. 2006. [http://www.vivo.colostate.edu/hbooks/pathphys/digestion/largegut/index.html The large intestine: Introduction and index]. ''Colorado State''. Retrieved July 1, 2007.
+
* Lichtenstein, G. R., P. D. Park, W. B. Long, G. G. Ginsberg, and M. L. Kochman. 1998. [http://www.ncbi.nlm.nih.gov/pubmed/9934753 Use of a push enteroscope improves ability to perform total colonoscopy in previously unsuccessful attempts at colonoscopy in adult patients] ''The American Journal of Gastroenterology'' 94(1): 187. PMID 9934753. Retrieved March 28, 2017.
−
* Bowen, R. 2000. [http://www.vivo.colostate.edu/hbooks/pathphys/digestion/largegut/anatomy.html Gross and microscopic anatomy of the large intestine]. ''Colorado State''. Retrieved July 1, 2007.
+
* Nall, Rachel. 2015. [http://www.healthline.com/health/redundant-colon Redundant Colon] ''MayoClinic.com''. Retrieved March 28, 2017.
−
 
 
−
==Additional images==
 
−
<gallery><gallery>
 
−
Image:illu_intestine.jpg|Intestines
 
−
Image:Dickdarm-Schema.svg|Scheme
 
−
</gallery>
 
  
 
==External links==
 
==External links==
−
* [http://stomastomata.com/ colon cancer experience]
+
All links retrieved January 7, 2024.
−
* [http://training.seer.cancer.gov/module_anatomy/unit10_3_dige_region4_intestine.html Overview and diagrams at seer.cancer.gov]
+
* [https://www.verywell.com/im-only-19-could-i-have-colon-cancer-as-a-teenager-796614 Colon Cancer in Teens and Young Adults] ''www.verywell.com''
−
* {{MerckHome|09|118|h}}
+
----
−
* {{MeshName|Large+Intestine}}
 
−
* {{IowaHistologyInteractive|393}}
 
−
* {{SUNYAnatomyLabs|37|13|01|00}} - "Abdominal Cavity: The Colon and its Divisions"
 
−
* [http://bellsouth.healthology.com/emb_player/embedad.aspx?content_id=3843&focus_handle=colon-cancer&brand_name=bellsouth Video: What is Colorectal Cancer?]
 
−
 
 
 
{{Digestive tract}}
 
{{Digestive tract}}
  

Latest revision as of 22:38, 7 January 2024

Colon (anatomy)
Intestine.png
Gray1223.png
Front of abdomen, showing surface markings for liver, stomach, and large intestine.
Gray's subject #249 1177
Dorlands/Elsevier c_47/12249855

The colon is the longest portion of the large intestine of vertebrates; in mammals, this section of the gastrointestinal tract extends from the cecum to the rectum. The colon often is incorrectly used in the meaning of the whole large intestine altogether; it is really only the biggest part of the large intestine. A storage tube for solid wastes, the main function of the colon appears to be extraction of water and salts from feces.

While chemical digestion does not take place in the colon—there are no digestive enzymes produced after the small intestine—there is a symbiotic relationship with bacteria that helps break down fiber, producing some nourishment for the body. Herbivores (like horses and rabbits), which depend on microbial fermentation, have a larger colon than omnivores (like pigs and humans), which in turn is larger than that of carnivores (such as dogs and cats) (Bowen 2000).

Anatomy

The large intestine is the terminal, tubular portion of the gastrointestinal tract (gut) of vertebrates prior to the anus or cloaca. Three major portions of the large intestine generally are recognized in mammals: caecum (blind-ended pouch), colon (majority of the length of the intestine), and rectum (short, terminal segment) (Bowen 2000). The location of the parts of the colon are either in the abdominal cavity or behind it in the retroperitoneum. The colon in those areas is fixed in location.

Arterial supply to the colon of humans comes from branches of the superior and inferior mesenteric arteries. Flow between these two systems communicates via a "marginal artery" that runs parallel to the colon for its entire length. Historically, it has been believed that the arc of Riolan, or the meandering mesenteric artery (of Moskowitz), is a variable vessel connecting the proximal superior mesenteric artery (SMA) to the proximal inferior mesenteric artery (IMA). However, recent studies conducted with improved imaging technology have questioned the actual existence of this vessel, with some experts calling for the abolition of the terms from future medical literature.

Venous drainage usually mirrors colonic arterial supply, with the inferior mesenteric vein draining into the splenic vein, and the superior mesenteric vein joining the splenic vein to form the portal vein, which then enters the liver.

Lymphatic drainage from the entire colon and proximal two-thirds of the rectum is to the paraortic nodes, which then drain into the cisterna chyli. The lymph from the remaining rectum and anus can either follow the same route, or drain to the internal illiac and superficial inguinal nodes. The dentate line only roughly marks this transition.

Portions of the human colon

Portions of the human colon (1, ascending colon; 2, traverse colon; 3, descending colon; 4, sigmoid colon)

In mammals, the colon consists of the ascending colon, transverse colon, and the descending colon, and in some mammals, including humans, a fourth section, the sigmoid colon. The colon from the cecum to the splenic flexure (the junction between the transverse and descending colon) is also known as the right colon. The remainder is known as the left colon.

Ascending colon

The ascending colon, on the right side of the abdomen, is about 12.5 centimeters long in humans. It is the part of the colon from the cecum to the hepatic flexure (the turn of the colon by the liver). It is retroperitoneal in most humans; that is, in the anatomical space behind (retro) the abdominal cavity. In ruminant grazing animals, the cecum empties into the spiral colon.

Anteriorly, the ascending colon is related to the coils of small intestine, the right edge of the greater omentum, and the anterior abdominal wall. Posteriorly, it is related to the iliacus, the iliolumbar ligament, the quadratus lumborum, the transverse abdominis, the diaphragm at the tip of the last rib, the lateral cutaneous, ilioinguinal, and iliohypogastric nerves, the iliac branches of the iliolumbar vessels, the fourth lumbar artery, and the right kidney.

The ascending colon is supplied by parasympathetic fibers of the vagus nerve (CN X).

Arterial supply of the ascending colon comes from the ileocolic artery and right colic artery, both branches of the SMA. While the ileocolic artery is almost always present, the right colic can be absent in five to fifteen percent of individuals.

Transverse colon

The transverse colon is the part of the colon from the hepatic flexure (the turn of the colon by the liver) to the splenic flexure (the turn of the colon by the spleen). The transverse colon hangs off the stomach, attached to it by a wide band of tissue called the greater omentum. On the posterior side, the transverse colon is connected to the posterior abdominal wall by a mesentery known as the transverse mesocolon.

The transverse colon is encased in peritoneum, and is therefore mobile (unlike the parts of the colon immediately before and after it). More cancers form as the large intestine goes along and the contents become more solid (water is removed) in order to form feces.

The proximal two-thirds of the transverse colon is perfused by the middle colic artery, a branch of superior mesenteric artery (SMA), while the latter third is supplied by branches of the inferior mesenteric artery (IMA). The "watershed" area between these two blood supplies, which represents the embryologic division between the midgut and hindgut, is an area sensitive to ischemia.

Descending colon

The descending colon is the part of the colon from the splenic flexure to the beginning of the sigmoid colon. It is retroperitoneal in two-thirds of humans. In the other third, it has a (usually short) mesentery. Arterial supply comes via the left colic artery.

Sigmoid colon

Stomach colon rectum diagram.svg

The sigmoid colon is the part of the large intestine after the descending colon and before the rectum. The name sigmoid means S-shaped. The walls of the sigmoid colon are muscular and contract to increase the pressure inside the colon, causing the stool to move into the rectum.

The sigmoid colon is supplied with blood from several branches (usually between two and six) of the sigmoid arteries, a branch of the IMA. The IMA terminates as the superior rectal artery.

Sigmoidoscopy is a common diagnostic technique used to examine the sigmoid colon.

Redundant colon

One variation on the normal anatomy of the colon occurs when extra loops form, resulting in a longer than normal organ. This condition, referred to as redundant colon, typically has no direct major health consequences, though rarely volvulus occurs resulting in obstruction and requiring immediate medical attention (Nall 2015).

A significant indirect health consequence is that use of a standard adult colonoscope for endoscopic examination is difficult and in some cases impossible when a redundant colon is present, though specialized variants on the instrument (including the pediatric variant) are useful in overcoming this problem (Lichtenstein et al. 1998).

Function

Although there are differences in the large intestine between different organisms, the large intestine is mainly responsible for storing waste, reclaiming water, maintaining the water balance, and absorbing some vitamins, such as vitamin K.

By the time the chyme has reached this tube, almost all nutrients and 90 percent of the water have been absorbed by the body. At this point some electrolytes like sodium, magnesium, and chloride are left as well as indigestible carbohydrates known as dietary fiber. As the chyme moves through the large intestine, most of the remaining water is removed, while the chyme is mixed with mucus and bacteria known as gut flora, and becomes feces. The bacteria break down some of the fiber for their own nourishment and create acetate, propionate, and butyrate as waste products, which in turn are used by the cell lining of the colon for nourishment. This is an example of a symbiotic relationship and provides about one hundred calories a day to the human body.

The large intestine produces no digestive enzymes—chemical digestion is completed in the small intestine before the chyme reaches the large intestine. The pH in the colon varies between 5.5 and 7 (slightly acidic to neutral).

Pathology

Intestines

There are a number of diseases or disorders of the colon:

  • Angiodysplasia of the colon
  • Chronic functional abdominal pain
  • Colitis
  • Colon cancer
  • Constipation
  • Crohn's disease
  • Diarrhea
  • Diverticulitis
  • Diverticulosis
  • Hirschsprung's disease (aganglionosis)
  • Intussusception
  • Irritable bowel syndrome
  • Polyposis (see also Colorectal polyp)
  • Pseudomembranous colitis
  • Ulcerative colitis and toxic megacolon

References
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External links

All links retrieved January 7, 2024.

Anatomy of torso, digestive system: Gastrointestinal tract
Upper GI
To stomach Mouth • Pharynx (nasopharynx, oropharynx, hypopharynx) • Esophagus (UES, LES)  • Crop
Stomach rugae – gastric pits – cardia/gland – fundus/gland – pylorus/gland – pyloric antrum – pyloric canal – greater curvature – lesser curvature – angular incisure
Lower GI
Intestine: small Duodenum (Suspensory muscle, Major duodenal papilla, Minor duodenal papilla) • Duodenojejunal flexure • Jejunum • Ileum • Ileocecal valve
continuous (intestinal villus, crypts of Lieberkühn, circular folds)
Intestine: large Vermiform appendix • Cecum • Colon (ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, sigmoid colon)
continuous (taenia coli, haustra, epiploic appendix)
Termination Rectum: Houston valve • rectal ampulla • pectinate line

Anal canal: anal valves • anal sinuses • anal columns • Hilton's white line

Anus: Sphincter ani internus muscle • Sphincter ani externus muscle
Lymph GALT: Peyer's patches (M cells)

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